The present invention relates to an optical multiplexer/demultiplexer having an arrayed waveguide grating, and, more particularly, to an optical multiplexer/demultiplexer capable of correcting a deviation of the wavelength of multiplexed/demultiplexed optical signal.
In optical wavelength (optical frequency) division multiplexing systems simultaneously transmitting signal beams having different wavelengths (frequencies) set at narrow wavelength intervals to thereby significantly increase the transmission capacity of optical transmission line, a plurality of optical signals are multiplexed by means of the multiplexing function of an optical multiplexer/demultiplexer on the transmitter side and wavelength-multiplexed light signal beam is demultiplexed to original optical signals by the demultiplexing function of an optical multiplexer/demultiplexer on the receiver side. This type of optical multiplexer/demultiplexer uses a grating to demultiplex wavelength-multiplexed light signal beams. Due to the limited diffraction order, however, the conventional grating can not sufficiently disperse wavelength-multiplexed light signal beam and has a difficulty in demultiplexing wavelength-multiplexed light signal beam into a plurality of optical signals when the wavelength intervals of the optical signals are narrow. In this respect, for example, an optical multiplexer/demultiplexer using an arrayed waveguide grating is used in the multiplexing of optical signals and demultiplexing of multiplexed light signal beam.
As shown in FIG. 4, this type of optical multiplexer/demultiplexer has slab waveguides 110 and 112 connected to both ends of an arrayed waveguide grating 107, so that wavelength-multiplexed light signal beam 200 input from an input waveguide 103 is demultiplexed into a plurality of optical signals which are taken out of a plurality of output waveguides 105. Reference numeral 101 indicates a substrate.
Specifically, the input-side slab waveguide 110 is comprised of a two-dimensional waveguide which has an effect of optical confinement in the vertical direction, and has an input end face (end face on the input waveguide side) and an output end face (end face on the grating side) thereof formed in arc shapes as viewed in a horizontal plane. The center of curvature 110y of the output end face of the input-side slab waveguide 110 coincides with the center 110y of the input end face of the slab waveguide 110. The input waveguide 103 is connected to the input end face of the input-side slab waveguide 110 at the center of curvature 110y. The input ends of a plurality of channel waveguides 107a which constitute the arrayed waveguide grating 107 are connected to the output end face of the slab waveguide 110 at intervals in the widthwise direction of the output end face.
The wavelength-multiplexed signal beam introduced into the input-side slab waveguide 110 from the input waveguide 103 is dispersed in the input-side slab waveguide. Dispersed optical signal waves reach the output end face of the slab waveguide 110 at the equal phase. In FIG. 4, reference numeral 210 indicates the equi-phase plane.
The output-side slab waveguide 112 is comprised of a two-dimensional waveguide which demonstrates optical confinement in the vertical direction, and has an input end face (end face on the grating side) and an output end face (end face on the output waveguide side) thereof formed in arc shapes as viewed in a horizontal plane. The center of curvature O of the input end face coincides with the center O of the output end face. The further outward a waveguide in the plurality of channel waveguides 107a is located, the longer the waveguide length becomes. The output ends of the channel waveguides 107a are connected to the input end face of the output-side slab waveguide 112 at intervals in the widthwise direction of the input end face.
As the waveguide lengths of the channel waveguides 107a of the grating 107 differ from one another, the optical signal waves that have propagated in the respective channel waveguides 107a have different phases from one another at the output of the grating 117. These optical signal waves are dispersed in the output-side slab waveguide 112 and are focused on the widthwise positions different according to the wavelengths on the output end face of the output-side slab waveguide.
The arrayed waveguide grating type optical multiplexer/demultiplexer satisfies the following equation (1).
nsxc2x7Dxc2x7sin xcfx86112+ncxc2x7xcex94L=mxcexxe2x80x83xe2x80x83(1)
where the symbol xcfx86112 represents the diffraction angle of light in the output-side slab waveguide 112, xcex is the wavelength, ns is the refractive index of the slab waveguide 112, nc is the refractive index of the channel waveguides 107a, xcex94L is the difference between waveguide lengths of the adjacent channel waveguides 107a, D is the interval of the channel waveguides 107a on the input end face of the slab waveguide 112, and m (integer) is the diffraction order.
The optical signal wave having the wavelength (center wavelength xcexM) thereof exhibiting the diffraction angle xcfx86112 of zero has an equi-phase plane 220a which extends along the input end face of the output-side slab waveguide 112. Because the input end face of the slab waveguide 112 is formed in an arc whose center of curvature is located at the center O of the output end face of the slab waveguide 112, the optical signal having the center wavelength xcexM is focused on the center of the output end face. The optical signal having the wavelength (center wavelength xcexM) thereof exhibiting the diffraction angle xcfx86112 (xe2x89xa00) has an equi-phase plane 220 inclined by an angle xcfx86112 counterclockwise in FIG. 4 with respect to the equi-phase plane 220 a associated with the center wavelength xcexM. Therefore, this optical signal is focused on a position P on the output end face of the output-side slab waveguide 112, which position is shifted in the widthwise direction from the center O of the output end face. That is, the optical signal focusing position P changes depending on the diffraction angle xcfx86112. In other words, the distance X between the center O of the output end face of the output-side slab waveguide 112 and the focusing position P differs according to the wavelength xcex.
When the diffraction angle xcfx86112 (hereinafter referred to as xe2x80x9cxcfx86xe2x80x9d) is small, sin xcfx86≈xcfx86, so that modifying the equation (1) yields the following equation.
nsxc2x7Dxc2x7xcfx86+ncxc2x7xcex94L=mxc2x7xcex
Solving this equation with respect to xcfx86, we obtain
xcfx86=(mxc2x7xcexxe2x88x92ncxc2x7xcex94L)/(nsxc2x7D).
Differentiating both sides of the above equation with respect to xcex yields
(dxcfx86/dxcex)=[{mxe2x88x92(dnc/dxcex)xc2x7xcex94L}xc2x7nsxe2x88x92{mxc2x7xcexxe2x88x92ncxc2x7xcex94L)xc2x7(dns/dxcex)}]/{(ns)2xc2x7D}.
In the vicinity of the center wavelength xcexM, the following is satisfied.
xcex=xcexM=(ncxc2x7xcex94L)/m
Thus, the following relationship is met.
xcex94L=(mxc2x7xcexM)/nc
Substituting the above relationship in the right-hand side of the above equation about (dxcfx86/dxcex), yielding
(dxcfx86/dxcex)=(xcex94L/nsxc2x7Dxc2x7xcexM)xc3x97{ncxe2x88x92xcexMxc2x7(dnc/dxcex)}
By expressing the radius of curvature of the input end face of the output-side slab waveguide as fo, the dispersion dX/dxcex of the distance X with respect to the wavelength xcex is given by the following equation (2).                                                                                           ⅆ                  X                                /                                  ⅆ                  λ                                            =                                                f                  O                                ·                                  (                                                            ⅆ                      φ                                        /                                          ⅆ                      λ                                                        )                                                                                                        =                                                {                                                            (                                                                                                    f                            O                                                    ·                          Δ                                                ⁢                                                  xe2x80x83                                                ⁢                        L                                            )                                        /                                          (                                                                        n                          S                                                ·                        D                        ·                                                  λ                          M                                                                    )                                                        }                                xc3x97                                  {                                                            n                      C                                        -                                                                  λ                        M                                            ·                                              (                                                                              ⅆ                                                          n                              C                                                                                /                                                      ⅆ                            λ                                                                          )                                                                              }                                                                                        (        2        )            
The above equation (2) indicates that the demultiplexed optical signal whose wavelength differs from the center wavelength xcexM by dxcex is taken out of the output waveguide 105 connected to the output-side slab waveguide at the position apart by the distance dX from the center O of the output end face of the output-side slab waveguide.
However, the core width and refractive index of each channel waveguide may differ from the designed values due to the production errors. In this case, the center wavelength of optical signal to be multiplexed/demultiplexed deviates from the designed value, raising such a problem that the wavelengths of the optical signals to be taken out of a plurality of output waveguides of the optical multiplexer/demultiplexer deviates from the desired wavelengths.
The arrayed waveguide grating type optical multiplexer/demultiplexer has such a characteristic that the loss is very small at the time of multiplexing/demultiplexing optical signal whose wavelength lies in a narrow wavelength range, raising such a problem that when the center wavelength of multiplexed/demultiplexed optical signal deviates from the desired wavelength, a loss occurs at the time of multiplexing/demultiplexing optical signal so that the optical signal is attenuated while multiplexing/demultiplexing is repeated.
To overcome such a problem, a technique is proposed which corrects the deviation of the center wavelength using an input waveguide and output waveguide for correction, respectively connected to the positions shifted from the center of the input end face of the input-side slab waveguide (the center of curvature of the output end face) and the center of the output end face of the output-side slab waveguide (the center of curvature of the input end face). (See Japanese Unexamined Patent Publication (KOKAI) No. 9-49936.) According to this technique, when the focusing position of optical signal with the center wavelength is shifted to P from O in FIG. 4, for example, the wavelength-multiplexed optical signal is input to the slab waveguide 110 via the correction input waveguide 103 connected to the position shifted from the center of the input end face of the input-side slab waveguide 110 as indicated by the two-dot chain line in FIG. 4 and the wavelength-multiplexed light is dispersed in the slab waveguide 110. An equi-phase plane 210a (indicated by the two-dot chain line in FIG. 4) of the dispersed optical signal is inclined by the angle xcfx86112 clockwise in FIG. 4 with respect to the equi-phase plane 210 in the case where wavelength-multiplexed optical signal is input to the center 110y of the input end face. At the output of the arrayed waveguide grating 107, therefore, the optical signal that has the center wavelength xcexM has the equi-phase plane 220a inclined by the angle xcfx86112 clockwise with respect to the equi-phase plane 220 in the case where wavelength-multiplexed optical signal is input to the center of the input end face of the input-side slab waveguide 110, and is thus focused on the center O of the output end face of the output-side slab waveguide 112.
As described above, the proposed technique provides a correction amount enough to cancel out the deviation of the center wavelength, originated from the deviation of the equi-phase plane of the optical signal wave that propagates in the output-side slab waveguide, at the incident position of wavelength-multiplexed optical signal and hence at the equi-phase plane of the optical signal wave that propagates in the input-side slab waveguide, thereby correcting the deviation of the center wavelength of the multiplexed/demultiplexed optical signal (the deviation of the condensing position of the optical signal wave that has the center wavelength).
While the amount of the deviation of the center wavelength takes an arbitrary value which depends on the production errors of the optical multiplexer/demultiplexer, however, the interval between the locations of the correction input and output waveguides is limited. That is, the intervals between the correction waveguides and the reference waveguide and the interval between the correction waveguides should be made larger than the allowable minimum interval that prevents the crosstalk characteristic from being degraded by coupling (crosstalk) of optical signals respectively transmitted through adjacent waveguides. According to the proposed technique, therefore, the minimum amount of correction of the deviation of the center wavelength is limited by the interval between the adjacent waveguides that is least necessary to avoid the degradation of the crosstalk characteristic caused by the optical coupling between the waveguides, thus disabling the optimal correction of the deviation of the center wavelength.
It is therefore an object of the present invention to provide an arrayed waveguide grating type optical multiplexer/demultiplexer which can ensure the optimal correction of the deviation of the wavelength of multiplexed/demultiplexed optical signal by eliminating the restriction to the amount of correction of the wavelength deviation while securing the interval between waveguides that can avoid optical coupling between the waveguides, and can thus significantly reduce the wavelength deviation.
To achieve the above object, this invention provides an optical multiplexer/demultiplexer having a substrate on which an arrayed waveguide grating comprising a plurality of waveguides having waveguide lengths different from one another, first and second slab waveguides respectively connected to both ends of the grating, a plurality of first waveguides adapted to output optical signal toward the first slab waveguide and a plurality of second waveguides adapted to receive optical signals output from the second slab waveguide are formed. The optical multiplexer/demultiplexer is characterized in that at least one of the plurality of first waveguides has a slab-waveguide-side end face thereof placed apart from a first-waveguide-side end face of the first slab waveguide and obliquely extending at an acute angle to an optical axis of the first waveguide as viewed in a plane.
According to this invention, the optical signal that is output from the first waveguide having an oblique end face apart from the end face of the first slab waveguide is refracted at the oblique end face and travels in the oblique direction toward the first slab waveguide. The signal beams from the first waveguide enter the end face of the first slab waveguide at the position close to or apart from another first waveguide as seen in the widthwise direction of the first slab waveguide from the position where the optical axis of the first waveguide intersects the end face of the first slab waveguide. The incident position of the optical signals changes mainly in accordance with the distance between the oblique end face of the first waveguide and the opposing end face of the first slab waveguide, the angle formed by the oblique end face and the optical axis of the first waveguide, the refractive index of the optical transmission medium between the oblique end face of the first waveguide and the opposing end face of the first slab waveguide, and the refractive index of the first waveguide, which indexes define the incident angle and the refraction angle at the oblique end face.
Anyway, the optical signal incident positions to the end face of the first slab waveguide from the first waveguide having the oblique end face are deviated from the position where the optical axis of the first waveguide intersects the end face of the first slab waveguide. Therefore, the interval between the optical signal incident position to the first slab waveguide from the first waveguide having the oblique end face and the optical signal incident position to the first slab waveguide from another first waveguide differs from the interval between two intersection positions at which the optical axes of the first waveguides and the end face of the first slab waveguide intersect (the disposition interval between those first waveguides at the end face of the first slab waveguide).
In the optical multiplexer/demultiplexer, the optical signals that enter the second slab waveguide from the first waveguides via the first slab waveguide and the arrayed waveguide grating are focused on positions on the second-waveguide-side end face of the second slab waveguide which differ from one wavelength to another. These focused positions vary also in accordance with the optical signal incident positions to the waveguide-side end face of the first slab waveguide from the first waveguides. In other words, at the time optical signals are incident to the first slab waveguide, a deviation of the wavelength of the optical multiplexer/demultiplexer can be corrected by using, instead of using the first waveguide having the oblique end face, another first waveguide or using the former one in place of the latter one, so that the interval between the light incident positions to the first slab waveguide from the first waveguide having the oblique end face and another first waveguide corresponds to the amount of correction of the center wavelength in the optical multiplexer/demultiplexer.
That the interval between the optical signal incident positions to the first slab waveguide from these two first waveguides differs from the disposition interval between both the waveguides indicates that the amount of correction of the center wavelength is not restricted by the disposition interval between the waveguides. That is, the amount of correction of the center wavelength deviation can be made smaller or greater than the value that corresponds to the disposition interval between the first waveguides by properly setting the distance between the oblique end face of the first waveguide and the end face of the first slab waveguide and the angle at which the oblique end face extends. In this manner, according to this invention, the amount of correction of the center wavelength can be set without a restriction by the disposition interval between the first waveguides. It is therefore possible to optimally correct a deviation of the center wavelength and provide a low-loss optical multiplexer/demultiplexer with a high yield.
Preferably, the plurality of first waveguides include a plurality of first waveguides for correcting a wavelength deviation, provided on both sides of the first waveguide serving as a reference as seen in the widthwise direction of the first-waveguide-side end face of the first slab waveguide. With this preferred arrangement, in a case where the wavelength becomes higher or lower than the desired wavelength due to the wavelength deviation, the wavelength deviation can be corrected by using the proper one of the plurality of first waveguides for correcting a wavelength deviation.
According to this invention, the at least one first waveguide for correcting a wavelength deviation has a slab-waveguide-side end face placed apart from the first-waveguide-side end face of the first slab waveguide and obliquely extending at an acute angle to the optical axis of the first waveguides as viewed in a plane. That side of the slab-waveguide-side end face of the first waveguide for correcting a wavelength deviation which is closer to the first waveguide serving as a reference as seen in the widthwise direction of the first slab waveguide is closer to the first-waveguide-side end face of the first slab waveguide as seen in the lengthwise direction of the first slab waveguide.
As already described, according to the conventional arrayed waveguide grating type optical multiplexer/demultiplexer, the interval between the optical signal incident positions from the reference and correction input waveguides, i.e., the amount of correction of the center wavelength is restricted by the disposition interval between the input waveguides. This makes it difficult to reduce the minimum amount of correction of the deviation of the center wavelength from the viewpoint of preventing the deteriorated optical multiplexing/demultiplexing characteristics caused by optical coupling between input waveguides.
On the contrary, according to the preferred arrangement of this invention, even when the first waveguides are arranged at intervals wide enough to avoid the crosstalk interference in the optical multiplexing/demultiplexing characteristics, it is possible to adequately set the intervals between the optical signal incident position to the first slab waveguide from the reference first waveguide and the optical signal incident positions to the first slab waveguide from the correction first waveguides, thereby making sufficiently smaller the minimum value of the amount of correction of the inclination of the equi-phase plane of the optical signal wave that propagates in the first slab waveguide, whereby the minimum value of the amount of correction of the deviation of the center wavelength can be made sufficiently smaller. This can ensure optimal correction of the deviation of the center wavelength.
In the case where a plurality of first waveguides for correcting a wavelength deviation are provided, particularly, the amount of correction of the wavelength deviation attained by these first waveguides for correction can be substantially continuously varied by properly setting the distances between the first waveguides for correction and the first waveguide for reference as seen in the widthwise direction of the first slab waveguide, the distances between the first waveguides for correction and the end face of the first slab waveguide as seen in the lengthwise direction of the first slab waveguide, the angle of the oblique end face, etc. As a result, regardless of how much the amount of deviation of the center wavelength is, it is possible to significantly reduce the deviation of the center wavelength to minimize the amount of the deviation, preferably substantially to zero by selecting the proper correction waveguide to minimize the amount of the wavelength deviation, preferably to make a correction to cancel out the wavelength deviation. This can improve the loss characteristic of the optical multiplexer/demultiplexer and the production yield.
Although the foregoing description has mainly explained the demultiplexing function to cause the optical signals (wavelength-multiplexed optical signals) to be incident from the first waveguides to the first slab waveguide and cause optical signals having different wavelengths to be taken out of a plurality of second waveguides, the optical multiplexer/demultiplexer of this invention also demonstrates the multiplexing function to cause optical signals to be incident to a plurality of second waveguides and cause wavelength-multiplexed optical signals taken out of the first waveguides, as apparent from the principle of reciprocity of this optical multiplexer/demultiplexer.